A printer generally uses a linefeed roller and an output roller to drive a medium in the printer during a printing process. The linefeed roller and the output roller are driven by a servo motor. A pick motor controls a pick system to pick up the medium, for example a paper, from an input tray and feeds it to the linefeed roller. The linefeed roller drives the paper into a printing area where droplets of ink are sprayed onto the paper from an ink cartridge.
One or more pinch rollers are biased against the linefeed roller so that the paper is driven between the pinch rollers and the linefeed roller. Since the pinch rollers are biased against the linefeed roller, a pinch force is exerted on the paper. The linefeed roller and the pinch rollers control the advancement of the paper during most of the printing process.
In some printing processes, once a bottom of form (BOF) edge of the paper leaves the linefeed roller and the pinch rollers (the pinching point), the output roller drags the paper from the printing area to an output tray. One or more star wheels are normally used together with the output roller to drag the paper from the printing area. The star wheels are located adjacent to the output roller, with the spikes of the star wheels touching the output roller. The paper is dragged out of the printing area between star wheels and the output roller.
The configuration of the printer described above allows the printer to continue to print on the paper even when the paper has left the pinching point. This enables the printed image on the paper to have very small BOF margin, or even full bleed printing.
However, paper positioning errors normally occur when the control of the driving of the paper is changed from the linefeed roller to the output roller. Such positioning errors are called BOF transition error (BOFTE). The BOFTE are more prominent in high quality photo printing. One of the main causes of BOFTE is the result of pinch rollers squeezing the bottom edge of the paper when the paper leaves the pinching point.
Special print mode may be applied during or after transition from the linefeed roller to the output roller to smoothen printing defects caused by BOFTE. It is also possible to use special print head swath shifting corresponding to the paper movement during the transition to minimize such printing defects. However, the printing defects caused by BOFTE still could not be eliminated using such methods, and these methods may also cause additional printing defects.
It is desirable to provide a method and a system to reduce BOFTE in small BOF margin and full bleed printing.